The measurement of humidity in inspired and expired gas would be helpful in the management of patients undergoing anesthesia and critical care medicine. Oxygen consumption is commonly measured during steady state conditions by invoking the Haldane transformation (mass balance of nitrogen over the respiratory cycle) to calculate the increase in expired volume due to increased temperature and added water vapor. However, during high inspired O.sub.2 breathing or during non-steady state conditions, mass balance for nitrogen cannot be used. Then, a direct measurement of humidity in inspired and expired gas, along with measurements of airway flow, O.sub.2 fraction, and temperature could generate a measurement of O.sub.2 consumption per breath (V.sub.O2,br).
During mechanical ventilation of patients during anesthesia or critical care medicine, breath-by-breath measurements of gas humidity at the airway opening are rarely conducted. Current humidity sensors are bulky, respond slowly, and lack disposability due to their expense.
Other applications of a humidity sensor include:
1. Assessment of heat and water exchange during ventilation in anesthesia and critical care medicine, including the efficacy of humidifiers. Benefits of adequate inspired gas humidification include conservation of patient temperature, decreased airway mucosal drying (less inspissation of secretions and preservation of muco-ciliary transport and lung defense), and decreased cooling of airways (minimize provocation of bronchoconstriction). PA1 2. Assessment of aerosol deposition and/or penetration of airborne pollutants. Many pollutants and medicinal aerosols are hygroscopic (i.e. have affinity for water vapor). PA1 3. Airway thermometry measurement of cardiac output. The addition of airway humidity measurement may enhance the measurement of pulmonary blood flow by airway thermometry. PA1 4. Industrial applications, such as monitoring ambient atmospheres in clean rooms (e.g. microelectronics), require inexpensive, fast response, accurate, and full water vapor range measurement of humidity.
There have been prior attempts to develop a probe for in vivo measurement of airway humidity during anaesthesia. See for example the two-wire capacitive absorption sensor described by Farley et al. in "Development of a probe for the in vivo measurement of airway humidity during anaesthesia," J.Biomed.Eng. 1990 Vol. 12, July, 328-332. The probe is located 20 cm into the airway, corresponding to the level of the mid-trachea. See also Donaldson, "Methods for Measuring Sputum Viscosity and Inspired Air Humidity in Tracheostomized Patients," Nursing Research, Vol. 17, No. 5, pp 388-395, wherein there is described a portable "pistol" and temperature indicating instrument in which a small fan draws air over two thermistors in a barrel, one of which is covered with a wet wick. A minimum of 30 seconds was required for each temperature reading.
The humidification of dry gas presented to the ventilated patient on an intensive therapy unit or during anaesthesia has been advocated by the medical profession for many years; see Tilling et al. "An accurate method of measuring medical humidifier output" Clin. Phys. Physiol Meas., 1983, Vol. 4, No. 2, 197-209, where the authors used a capacitance hygrometer.
The concept of differential wet-bulb and dry-bulb response in thermocouple physchrometry is well-established. Shaw et al. in "The Effect of and Correction for Different Wet-Bulb and Dry-Bulb Response in Thermocouple Physchrometry," J.of App'd Meteorology, Vol. 19, January 1980, 90-97 describes a wet-bulb sensor fabricated by wrapping one of the thermocouples with wicking compound obtained by separating the strands of cotton sewing thread. Water was supplied from a small reservoir while careful control of the flow rate was provided by "intravenous bag" flow rate adjustment on the tubing.
Powell in "The Use of Thermocouples for Psychrometric Purposes," Proc. Phys.Soc., Vol. 48, 406-414 describes the use of a thermocouple wrapped with very fine cotton for a distance of a centimeter on each side of the junction.
Ingelstedt in "Humidifying Capacity of the Nose," Ann. Otol.Rhinol.Laryngol. Vol. 79: 475-480 describes keeping a thermoelement moist using hygroscopic fibers.
Martin et al. in "Comparing Two Heat and Moisture Exchangers with One Vaporizing Humidifier in Patients with Minute Ventilation Greater than 10 L/min," Chest Vol. 107, May 5, 1995, 1411-1415, describes measuring humidity at the Y-piece of an inspiratory and expiratory line. They describe the use of two thermal probes, one wet and one dry, the upstream probe measuring the gas temperature, the downstream probe coated by sterile cotton wet with sterile water.
Eisner et al. in "Design and development of a micro-thermocouple sensor for determining temperature and relative humidity patterns within an airstream" Journal of Biomechanical Engineering, November 1989, Vol. 111 283-287, describes the use of a wetted miniature thermocouple coated with borom nitride to act as a wicking material.
Ingelstedt, in "Studies on the Conditioning of Air in the Respiratory Tract," Acta Oto-Laryngologica Supplement, 1956, pp 1-87 and attachments, describes a psychrometer in which the thermocouple wires are butt welded. The wet side wire was wrapped with rayon silk fibers, wound not only around the junction, but also along the wires at both sides of the junction to provide a hydroscopic material for the junction.
Other references discussing the problem of psychrometric measurement include:
Tsukamoto, "Dynamic Response of the Fine Wire Psychrometer for Direct Measurement of Water Vapor Flux," Journal of Atmospheric and Oceanic Technology, Vol. 3, September 1986, 453-461.
Kalogiros et al. "Fast-Response Humidity Measurements with the Psychrometric Method," Journal of Applied Meteorology, Vol.32, September 1993, 1499-1507.
Davis, "Detailed Discussion of the Dry and Wet Bulf Psychrometer," Chapter III, ASHRAE Brochure on Psychrometry, ASHRAE Technical Committee on Psychrometrics (1969-1972) American Society of Heating Refrigerating and Air-Conditioning Engineers, Inc., 81-90.
Ingelstedt, "Aerodynamics Within the Larynx and Trachea," Acta oto-laryng. Suppl.158, 81-92.
Feihl, "The Temperature and Water Output of Heat and Moisture Exchangers During Synchronous Intermittent Mandatory Ventilation," Acta. Anaesth.Italica Vol.43, Suppl. 157-64, 1992.
Chiranda, "Evaluation of a 3rd Generation Heat and Moisture Exchanger as an Alternative to Conventional Humidifiers in ICU," Acta. Anaesth.Italica Vol. 43, Suppl. 173-76, 1992.
Jackson, "An Evaluation of the heat and moisture exchange performance of four ventilator circuit filters," Intensive Care Medicine (1992) Vol. 18, 264-268.
Brackenbury, "Measurement of water loss in exercising animals using an electronic humidity detector," Med. & Biol. Eng. & Comp. 1982, Vol. 20, 433-436.